Nuclear reactor control system



Feb. 23, 1965 s. oEs'rRElcl-IER 3,170,845

NUCLEAR REAcToR CONTROL SYSTEM original Filed Feb. 1v, 1955 5Sheets-Sheet l SEYMOUR OESTREICHER BY ATTORNEY Feb. 23, 1965 s.oEsrRElcHER 3,170,845

NUCLEAR REACTOR CONTROL SYSTEM Original Filed Feb. 17, 1955 5Sheets-Sheet 2 INVENTOR.

SEYMOUR O ESTREICHER A T TORNE Y Feb. 23, 1965 s. oEsTRElcHER 3,170,845

NUCLEAR REACTOR CONTROL SYSTEM Original -Filed Feb. 17, 1955 5Sheets-Sheet 3 IN VENTOR.

SEYMOUR OE STRE ICHER ATTORNEY Feb. 23, 1965 s. oEs'rRE-lcl-IER3,170,845

NUCLEAR REACTOR CONTROL SYSTEM Crlgnal Filed Feb. 17, 1955 5Sheets-Sheet 4 INVENTOR.

SEYMOUR OESTREICHER ATTORNEY Feb. 23, 1965 s. oEsTRElcHER 3,170,845

NUCLEAR REACTOR CONTROL SYSTEM original Filed Feb. 17, 1955 5sheets-sheet 5 INVENTOR SEYMOUR OESTREICHER ATTOR N EY United StatesPatent 3,170,845 NUCLEAR REACTR CONTROL SYSTEM Seymour Oestreicher,Mineola, N.Y., assignor to American Machine & Foundry Company, acorporation of New Jersey Continuation of application Ser. No. 488,808,Feb. 17,

1955. This application Nov. 29, 1961, Ser. No. 155,554 9 Claims. (C1.176-36) This invention relates to improvements in the control of nuclearreactions. More particularly, the invention relates to a novel system ofcontrol rods and co-ntrol rod driving means interchangeably mountedabout a reactor core.

This application is a continuation of my copending application SerialNo. 488,808, now abandoned, filed February 17, 1955, and entitledNuclear Reactor Control System.

In the operation of a nuclear reactor, neutron absorbing control rods,acting as adjustable brakes on the intensity of nuclear fission chainreaction, are moved in channels through a reactor core of fuel elementscontaining iissionable material by driving mechanisms mounted outside ofthe reactor core. Reactors are conventionally designed for particularoperations and use is limited to those operations originally intendedexcept where expensive and diticult changes in structure are made.Control rods and associated driving means are built to criticalspecifications depending precisely upon the use, neutron liuxdistribution, and power level for which a reactor is designed. When adifferent pattern of neutron ux is needed in the reactor core forexperimental purposes, for example, the location, pattern and number ofcontrol rods must be changed. Likewise, when a larger reaction iscontemplated than the reactor is designed to accommodate, an independentreactor must be built or major structural modifications undertaken. Thisoperation is awkward as it conventionally requires costly welding andcutting operations, often on radioactive material, and involvingsubstantially a complete redesign and rebuilding of the reactor. Thisineh'icient method of building reactors may alternatively require anexpensive series of different reactors to perform different kinds ofoperations.

Accordingly, one object of the present invention is to provide aflexible reactor and reactor control system which eliminates the needfor building several diiferent kinds of reactors to accommodatedifferent fuel loads.

Another object of the present invention is to provide a flexible reactorprovided with a novel flexible system ,of controls wherein differentcontrol rod loads and speeds can be controlled by a control rod drivingmechanism characterized by interchangeable gears and motors of varioussizes.

A further object of this invention is to provide for the accommodationwithin a given nuclear reactor of varying numbers of control rods,control rod driving mechanisms and fuel elements in relation to the sizeand kind of nuclear reaction to be controlled. Such controls can beselectively mounted and arranged in any of several different positionsaround an adjustable reactor core of iissionable material and individualcontrols and fuel elements can easily be added to or removed from thesystem.

Another object is to provide a novel method of controlling nuclearreactions.

The present invention comprises a nuclear reactor core through whichcontrol rods are moved by control rod driving means. This inventionembraces the interchangeable use and adjustment of one or more controlrods and auxiliary driving mechanisms, in conjunction with a number offuel elements, to adjust the capacity of the entire system or change itsoperating characteristics.

An advantage of this invention is that the number and type of drivingmechanisms can be selected according to the size of the reaction whichmust be controlled. Moreover, the invention includes the adaptabledistribution of control rods in different spaced relationships to oneanother and to the reactor core depending upon the particular reactioncontemplated. These relationships must be separately computed for eachsize or kind of reaction and when known, by relatively simple changes,either the control rods or the fuel elements or both in combination, canbe readily varied to satisfy the newrequirements of the reactor.

Another advantage of this invention is that different kinds of controlrod driving mechanisms may be selected, based upon theinterchangeability of parts, including motors and gears within thecontrol rod driving mechanism assembly, so that control rods can beVdriven at varying speeds, with varying degrees of precise control, andto carry different power loads. Concomitant interchangeable reactorinstrumentation and core structure complete the complement of novelinterchangeable features of this invention.

The foregoing objects and advantages will be realized and the inventionwill be more fully describedA by reference to the accompanying drawing.Y

Similar reference characters are applied to similar elements throughoutthe drawing.

FIGURE l is a perspective view of a swimming pool type nuclear reactorupon which is mounted a movable bridge and core support, parts beingshown broken away to reveal details of the apparatus.

FIGURE 2 is a detail view of a heterogeneous reactor core mounted on agrid plate in a swimming pool type reactor and showing the mounted corechannels of special hollow fuel elements which provide openings in thecore in which control rods move to meet any condition of operationdesired. These special elements are readily mounted in any selectedposition on the grid plate to receive control rods whose laterallocation is determined by the drive mechanism mounting shown in FIGURE3.

FIGURE 3 is a view of four controlV rod driving'mechanisms selectivelypositioned on a perforate mounting board which acts as a supportingmember with several mounting stations. v

FIGURE 4 is a partially exploded view of a preferred form of componentsof a control rod driving mechanism.

FIGURE 5 is a diagrammatic cross sectional view of a homogeneous waterboiler type reactor.

A unitized reactor may be built according to this invention in manyforms. Heterogeneous reactors such as the Bulk Shielding Facility orswimming pool, as well as the homogeneous or water boiler type reactormay be equally improved by design according to this invention.

In a preferred embodiment of the invention, as illustrated in FIGURE 1,there is shown a Bulk Shielding Facility type or swimming pool reactor,designed to perform large shielding experiments. In this construction,the reactor core 11 is suspended in a pool 13 which contains water inwhich radiation shielding can be set up 19. The core 11 is thensuspended from the bridge 19 asuitable distance, for example twentyfeet, below the surface of the water on a grid plate 21 by a suspensiontruss 23 which slips onto mountings 25 on the bridge 19. This detailappears in FIGURE 2.

In the form shown in FIGURES l and 3, mounting plate 27, already boredfor mounting control rods and associated drive mechanisms, is insertedand suitably secured or locked into place at the top of the suspensionstructure 23. This plate is shown in detail in FIGURE 3. A grid plate orsupport 21, shown in FIGURE 2, is used for mounting fuel elements 41.

FIGURE 4, which is an exploded View of a preferred embodiment of themotor package 29, or control rod driving mechanism, shows the basichousings with various interchangeable units.

The control rod drive includes: motors 2; main transmission unit withadapters 5; worm reducer -unit 6; coverplate 8; shim drive transmission10; magnetic clutch 12; regulating drive transmission 14; positiontransducers 16, such as potentiometers or synchro generators; main driveshaft rod guide housing 22; limit switch 24; switch actuator 26; drivenrack 2S; linear bearing 3G; and pinion 34.

The motor packages 29 bolt at anges or mounting pads 32 onto standardanchorages 31, as shown in FIG- URE 3, and are mounted on the bridge 19.All electrical connections from the motor packages 29, including thosefrom the position pickotls or position transducers 16, such aspotentiomcters or synchro generators, limit switches, magnet and motor2, for example, terminate in a suitable junction box (not shown) on themotor package mounting plate 27 A control console 33 containing all therequired standard nuclear measuring instruments can be mounted on thebridge 19, as shown in FIGURE 1, or in a separate control room. Thecontrols for the motor package 29 are plug-in units, although othertypes, such as screw-in kinds, also work. Each unit contains controlswitches, limit lights, position indicator and indicator drive. Theconsole has been prewired and tested. Plugging in control units andconnecting to added motor packages are the steps necessary to connectthem in operating order in the reactor operating and control system.

To load the reactor special hollow fuel elements 35, containing controlrods (not shown) and removable guide rods 37, are lowered into positionbelow the motor packages and attached to the grid plate, FIGURE 2. Thecontrol rods are aflxed to elongated rack driven supports 39. Once theelements 35 and guides 37 are in place, drive racks 28, extension andmagnetic clutches 12 are connected (not shown). Fuel elements 41 areinserted. The reactor is now ready for standard start up procedure. Allcomponents, including handling tools 36, used in this assembly arestandard unit components for which spare parts are easily kept insupply.

For a contemplated power level of 100 kilowatts and a iiux density of1012n/cm.2/sec. for example, three rods are required, two shim-safetyrods (coarse control), and one regulating rod (fine control). If anoperation is scheduled which requires a fuel loading greater than wasanticipated at the initial construction of the facility, then three rodsmay not be enough for safe operation and changes may have to be made.The unitized assembly has been designed for such contingencies. Anothershim safety rod can be added quickly and conveniently with no majorchanges in the system. A blank in the instrument console 33 is removed,revealing prewired connections. A standard control unit is inserted. Astandard motor package unit 29 is bolted into its predrilled locationand the drive rack Z8 and magnetic clutch 12 are inserted. The normalfuel elements 43 directly below are removed and replaced with the hollowspecial element 35 and guide rod 37. Electrical connections are made forthe fourth rod drive and additional fuel is loaded..

The reactor can now accommodate the new experiment and operate Withrequisite safety.

Another operation may come up.which requires the utilization of spacedirectly above and at the center of the lattice or reactor core assembly11. This space may be occupied by rods, guide or rack extensions or rodsupports 39. Once again the novel unitized method of controlling nuclearreactions permits a change in rod position which will result in a clearcentral space. The motor package mounting plate 27, in the form shown,is predrilled, FIGURE 3. The holes 31 are so arranged that a standardmounting is available for every point over the core lattice 11. Astandard drive lunit 29 can pick up its mounting hole 31 directly overany element and be bolted at a flange 32 connection. Since the specialfuel element 35 and rod guide 37 comprise an integral unit, it may beplaced in any location in the lattice 11. Therefore, after necessaryprecautions with regard to safetyhave been taken, the control rods andsupports 39, special elements 3S, and their guides 37 are moved to thecorners of the lattice 11, for example, and the ambulatory motorpackages 29 are unbolted at the flanges 32 and moved to new locations.Since the electrical connections are still intact Ithe new requirementhas been met with almost no effort. great advantage of unitizedcomponents is that flexibdity 1nherent in this approach allows a widerange of operations and also makes possible future experimentalarrangements for research not yet conceived.

The foregoing description of a swimming pool reactor is, of course, onlyone possible way of utilizing this invention. This invention is equallyapplicable to a homogeneous, water boiler type, nuclear reactor shown inFIGURE 5. In the water boiler type reactor, the core comprises asolution of a uranium salt in water contained in a sealed vessel 45,which is supported in the shielding chamber 13. Control rods 47 whichabsorb neutrons are suspended around this vessel in the same way ascontrol rods and supports 39 are suspended in a heterogeneous typereactor such as shown in FIGURE l. Control rods are moved by drivingmeans 29 past the surface of the homegenous reactor nuclear core vessel45, either around the vessel or through special control rod slots formedin the surface of the vessel. The movement of these control rods is inprinciple substantially the same as that of control rods in thedescribed heterogeneous type reactor. The core assumes a more permanentchar-l acter than that shown in the principal example where iexibledesign allows the shifting of core fuel elements 43 in conjunction withrearrangements of the control rods and supports 39.

The invention is also easily adapted to a sealed sys' tem type nuclearreactor such as may be convenient for a power producing system. In asealed system, the con'd trol mechanisms and core 11 are sealed from theoutside, such that all radioactive fluid is contained within a sealedstructure. When this invention is applied to a sealed system, the entireguide mechanism package as well as the control rods themselves must becontained within a sealed enclosure to prevent the escape of radioactivematerial. Even in a sealed system, control cornponents can be easilysubstituted for defective components or new, additional components canbe put into a system. Here again it is convenient to be able tosubstitute standard components.

There has thus been described the application of a system of controlmechanisms to various kinds of nuclear reactors and a method ofcontrolling nuclear reactions. Interchangeable control rods and controlrod driving mechanisms are flexibly mounted around a reactor core sothat they are easily relocated, removed and substituted for, or addedto. Convenient spacial rearrangement of control rods is provided for. Inone embodiment the number, size and location of fuel elements is alsoadjustable in combination with control rod relocation. This invention isadapted to use in a large number of kinds of nuclear reactors. Itprovides a iiexible system of growth so that nuclear reactors can bemodified to meet changing experimental needs. By means of this inventioncostly rearrangements and innovations in structure arev avoided. By useof the unitized system. of reactor con trol constituting the invention,the replacement of defective parts is easily accomplished. Therefore,this invention is a fundamental contribution to nuclear reactorconstruction and operation.

What is claimed is:

l. The method of arranging a swimming pool reactor having a system ofcontrol rod drive packages which can be readily rearranged as torelative locations and speeds of control with respect to immersedreactor core elements including the steps of:

(a) providing a core supporting grid located deep in the pool and havinga pattern including a multiplicity of identical core-element supportingstations shaped to receive and locate in definite mutually-parallelpositions plural clustered similarly-shaped core elements including fuelelements, at least some of said core elements having a verticle borereceiving a control rod, all of which control rods extend upwardly inparallel relationship from elements supported on said grid;

(b) providing plural control-rod drive packages having interchangeablemotor and gear means connected to actuate the reciprocable rod drives,and the packages having standardized housings with similarly drilledmounting anges located in standardized locations offset from theassociated reciprocable rod-drive and in a plane normal thereto;

(c) rigidly fixing a drive package mounting plate in a position abovethe pool and normal to said parallel control rods, and providing themounting plate with a multiplicity of standardized drive-packageanchorage positions drilled to register with the drilled flanges of anyrod drive package, and placing the anchorage positions in a definitepattern such that whenever a drive package is bolted in a selectedposition its reciprocable rod drive will be in alignment with acorresponding core supporting station of said grid;

(d) mounting a lattice of core elements clustered in selected stationson said grid with control rods located at certain stations;

(e) mounting a rod-drive package at each anchorage 'position whichcorresponds with one of said certain stations, and selecting andinstalling therein the motor and gear means necessary to drive theassociated control rod at a rate suitable to 4its intended controlsensitivity; and

(f) coupling each control rod with the reciprocable rod drive with whichit aligns.

2. The method of arranging a swimming pool reactor having a system ofcontrol rod drive packages which can be readily rearranged as torelative locations and speeds of control with respect to immersedreactor core elements including the steps of (a) providing a core and acore supporting grid located deep in the pool and having a patternincluding a multiplicity of identical core-element supporting stationsshaped to receive and locate in definite mutually-parallel positionsplural clustered similarlyshaped core elements including fuel elements,said core having vertical openings for receiving control rods, all ofwhich control rods extend upwardly in parallel relationship from thecore supportedon said slid;

(b) providing plural control-rod drive packages having interchangeablemotor and gear means connected to actuate reciprocable rod drives, andthepackages having standardized housings with mounting means which areall similar to each other and located in standardized locations offsetfrom the associated reciprocable rod-drive and in a plane normalthereto;

(c) rigidly fixing a drive package mounting plate in a position abovethe pool and normal to said parallel control rods, and providing themounting plate with a multiplicity of standardized drive-packageanchorage positions With holes shaped to cooperate with one of saidmounting means on a rod drive package, and placing the anchoragepositions in a definite pattern such that Whenever a drive package isanchored in a selected position its reciprocable rod drive will be inregister with a corresponding core supporting station of said grid;

(d) mounting a lattice of core elements clustered in selected stationson said grid with control rods located at certain stations;

(e) mounting a rod-drive package at an anchorage position whichcorresponds with one of said selected stations, and selecting andinstalling therein the motor and gear means necessary to drive theassociated control rod at a rate suitable to its intended controlsensitivity; and

(f) coupling each control rod with the reciprocable rod drive with whichit is in register.

3. In a nuclear reactor employing core congurations made up of multiplesimilarly shaped fuel elements, a certain critical number of which arerequired in a selected configuration vto sustain a nuclear reaction, andsaid reactor including control rods extending into bores in fuelelements for controlling said reaction and each rod being supported forreciprocation with respect thereto, said reactor including a grid havinga pattern of fuel-element mounting locations greatly exceeding saidcritical number and each shaped to receive and support a fuel-element ina precise location, and said reactor including a mounting plate disposedopposite to and spaced from said core supporting grid, the improvementcomprising: y

(a) a multiplicity of interchangeable control rod reciprocating driveunits each including a housing supporting a reciprocable rodsupport anda moto-r to drive the latter, the housings of all the units havingmounting flanges with holes therethrough in standardized locationsprecisely oriented with respect to said reciprocable rod support;

(b) said drive mounting plate having a number of identical mountingstations, each including a set of holes in standardized locations andsaid mounting stations being arranged in a pattern such that whenaligned with the holes of a drive package ange mounted thereon thereciprocable rod support will be in precise alignment with respect to afuel element location on said grid; and

(c) releasable securing means passed through the aligned holes forsecuring motor drive package mounting means in selected mountingstations of said plate.

4. A control system for a swimming pool reactor cornprising incombination:

(a) a nuclear core support immersed in water and disposed transverselyof an upwardly extending axis and having a pattern of possible fuelelement supporting positions in excess of the number of elementsrequired to sustain a nuclear reaction and said positions being allshaped the same to support complementarily Ashaped fuel element ends;

(b) fuel element-s occupying at least some of said positions andsupported in mutually parallel upright positions thereby, some of theelements having bores therein disposed parallel to said axis;

(c) plural control rods each disposed parallel with the axis and withone end extending into a bore of a fuel element;

(d) a mounting plate spaced along said axis at -a distance from saidcore support and rigidly lixed opposite thereto, said mounting platehaving a multiplicity of identical fastener stations each having pluralstandardized mounting holes occupying a definite position of registrywith respect to a different one of said fuel element supportingpositions;

(e) plural similar interchangeable rod-drive units each having areciprocable rod support connected with 7 and supporting a control rodfor reciprocating the latter with respect to the associated fuelelement, said drive units each comprising a unitary integral drivepackage including motor means coupled to drive transmission meansconnected to raise and lower said reciprocable rod support and saiddrive units all having mounting flanges with standardized mounting holeslocated to cooperate with the holes in any one of said fastener stationson said plate for positively locating the drive unit thereon to bringthe associated control rod support into registry with the bore in acorresponding fuel element on the core support; and

(f) releasable fastener means transxing aligned holes in the fastenerstations and the flanges.

5. In a control system as set forth in claim 4, each drive unit housingincluding standardized motor mounting means and standardizedtransmission mounting means for interchangeably receiving similarlyshaped motors and transmissions having different drive characteristics.

6. In a nuclear reactor employing core configurations made up ofmultiple similarly shaped fuel elements, a certain critical number ofwhich is required in a selected configuration to sustain a nuclearreaction, and said reactor including control rods extending intoopenings among the core fuel elements for controlling said reaction andeach rod being supported for reciprocation with respect thereto, saidreactor including a grid having a pattern of fuel-element mountinglocations greatly exceeding said critical number and each shaped toreceive and support a fuel-element in a precise location, and saidreactor including a mounting plate disposed opposite to and spaced fromsaid core supporting grid, the improvement comprising:

(a) a multiplicity of interchangeable control rod reciprocating driveunits each including a housing supporting a reciprocable rod support anda motor to drive the latter, the housings of all the units having firstmounting means arranged in mutually standardized locations preciselyoriented with respect to said reciprocable rod support;

(b) said drive mounting plate having a number of identical mountingstations, each including at least one hole shaped to cooperate with saidfirst mounting means and disposed in standardized locations and saidmounting stations being arranged in a pattern such that, when engagedwith said rst mounting means of a drive package so that said package ismounted thereon, the reciprocable rod support will be in preciseregister with an opening in the core located on said grid; and

(c) releasable securing means for securing each motor drive packagemounting means to a selected mounting station of said plate.

7. In a control system as set forth in claim 6, each drive unit housingincluding standardized motor mounting means and standardizedtransmission mounting means for interchangeably receiving similarlyshaped motors and transmissions having different drive characteristics.

S. A control system for a swimming pool reactor comprising incombination: l

(a) a nuclear core support immersed in water and disposed transverselyof an upwardly extending axis and having a pattern of possible fuelclement supporting positions in excess of the number of elementsrequired to sustain a nuclear reaction and said positions being allshaped 'the same to support complementarily shaped fuel element ends;

(b) fuel elements occupying at least some of said positions andsupported in mutually parallel upright positions thereby to form a core,said core having openings among the fuel elements therein disposedparallel to said axis;

(c) plural control rods each disposed parallel with the axis and withone end extending into one of said openings;

(d) a mounting plate spaced along said axis at a distance from said coresupport and rigidly xed opposite thereto, said mounting plate having amultiplicity of identical fastener stations each having at least onestandardized mounting hole occupying a definite position of registrywith respect to any one of said fuel element supporting positions;

(e) plural similar interchangeable rod-drive units each having areciprocable rod'support connected with and supporting a control rod forreciprocating the latter with respect to lthe core, said drive unitseach cornprising a unitary integral drive package including motor meanscoupled to drive transmission means connected to raise and lower saidreciprocable rod support, and Said drive units all having standarizedmounting means located to cooperate with the mounting holes in any oneof said fastener stations on said plate for positively locating thedrive unit thereon to bring the associated control rod support intoregistry with an opening in the core; and

(f) releasable fastener means for securing the drive units to themounting plate at selected stations.

9. In a control system as set forth in claim 8, each drive unit housingincluding standardized motor mounting means and standardizedtransmission mounting means for interchangeably receiving similarlyshaped motors and transmissions having different drive characteristics.

References Cited in the tile of this patent AECD-3425, AEC Document,April 15, 1950, pp. 1-7, 10, and 13.

Nucleonics, November 1952, pp. 56-60, vol. 10, No. 11.

Nuclear Engineering, CEP Symposium Series, vol. 50, No, 11, 1954, pp.203-212.

1. THE METHOD OF ARRANGING A SWIMMING POOL REACTOR HAVING A SYSTEM OFCONTROL ROD DRIVE PACKAGES WHICH CAN BE READILY REARRANGED AS TORELATIVE LOCATIONS AND SPEEDS OF CONTROL WITH RESPECT TO IMMERSEDREACTOR CORE ELEMENTS INCLUDING THE STEPS OF: (A) PROVIDING A CORESUPPORTING GRID LOCATED DEEP IN THE POOL AND HAVING A PATTERN INCLUDINGA MULTIPLICITY OF IDENTICAL CORE-ELEMENT SUPPORTING STATIONS SHAPED TORECEIVE AND LOCATE IN DEFINITE MUTUALLY-PARALLEL POSITIONS PLURALCLUSTERED SIMILARLY SHAPED CORE ELEMENTS INCLUDING FUEL ELEMENTS, ATLEAST SOME OF SAID CORE ELEMENTS HAVING A VERTICLE BORE RECEIVING ACONTROL ROD, ALL OF WHICH CONTROL RODS EXTGEND UPWARDLY IN PARALLELRELATIONSHIP FROM ELEMENTS SUPPORTED ON SAID GRID; (B) PROVIDING PLURALCONTROL-ROD DRIVE PACKAGES HAVING INTERCHANGEABLE MOTOR AND GEAR MEANSCONNECTED TO ACTUATE THE RECIPROCABLE ROD DRIVES, AND THE PACKAGESHAVING STANDARDIZED HOUSINGS WITH SIMILARLY DRILLED MOUNTING FLANGESLOCATED IN STANDARDIZED LOCATIONS OFFSET FROM THE ASSOCIATEDRECIPROCABLE ROD-DRIVE AND IN A PLANE NORMAL THERETO; (C) RIGIDLY FIXINGA DRIVE PACKAGE MOUNTING PLATE IN A POSITION ABOVE THE POOL AND NORMALTO SAID PARALLEL CONTROL RODS, AND PROVIDING THE MOUNTING PLATE WITH AMULTIPLICITY OF STANDARDIZED DRIVE-PACKAGE ANCHORAGE POSITIONS DRILLEDTO REGISTER WITH THE DRILLED FLANGES OF ANY ROD DRIVE PACKAGE, ANDPLACING THE ANCHORAGE POSITIONS IN A DEFINITE PATTERN SUCH THAT WHENEVERA DRIVE PACKAGE IS BOLTED IN A SELECTED POSITION ITS RECRPROCABLE RODDRIVE WILL BE IN ALIGNMENT WITH A CORRESPONDING CORE SUPPORTING STATIONOF SAID GRID; (D) MOUNTING A LATTICE OF CORE ELEMENTS CLUSTERED INSELECTED STATIONS ON SAID GRID WITH CONTROL RODS LOCATED AT CERTAINSTATIONS; (E) MOUNTING A ROD-DRIVE PACKAGE AT EACH ANCHORAGE POSITIONWHICH CORRESPONDS WITH ONE OF SAID CERTAIN STATIONS, AND SELECTING ANDINSTALLING THEREIN THE MOTOR AND GEAR MEANS NECESSARY TO DRIVE THEASSOCIATED CONTROL ROD AT A RATE SUITABLE TO ITS INTENDED CONTROLSENSITIVITY; AND (F) COUPLING EACH CONTROL ROD WITH THE RECIPROCABLE RODDRIVE WITH WHICH IT ALIGNS.
 3. IN A NUCLEAR REACTOR EMPLOYING CORECONFIGURATIONS MADE UP OF MULTIPLE SIMILARLY SHAPED FUEL ELEMENTS, ACERTAIN CRITICAL NUMBER OF WHICH ARE REQUIRED IN A SELECTEDCONFIGURATION TO SUSTAIN A NUCLEAR REACTION, AND SAID REACTOR INCLUDINGCONTROL RODS EXTENDING INTO BORES IN FUEL ELEMENTS FOR CONTROLLING SAIDREACTION AND EACH ROD BEING SUPPORTED FOR RECIPROCATION WITH RESPECTTHERETO, SAID REACTOR INCLUDING A GRID HAVING A PATTERN OF FUEL-ELEMENTMOUNTING LOCATIONS GREATLY EXCEEDING SAID CRITICAL NUMBER AND EACHSHAPED TO RECEIVE AND SUPPORT A FUEL-ELEMENT IN A PRECISE LOCATION, ANDSAID REACTOR INCLUDING A MOUNTING PLATE DISPOSED OPPOSITE TO AND SPACEDFROM SAID CORE SUPPORTING GRID, THE IMPROVEMENT COMPRISING: (A) AMULTIPLICITY OF INTERCHANGEABLE CONTROL ROD RECIPROCATING DRIVE UNITSEACH INCLUDING A HOUSING SUPPORTING A RECIPORCABLE ROD SUPPORT AND AMOTOR TO DRIVE THE LATTER, THE HOUSINGS OF ALL THE UNITS HAVING MOUNTINGFLANGES WITH HOLES THERETHROUGH IN STANDADRIZED LOCATIONS PRECISELYORIENTED WITH RESPECT TO SAID RECIPROCABLE ROD SUPPORT; (B) SAID DRIVEMOUNTING PLATE HAVING A NUMBER OF IDENTICAL MOUNTING STATIONS, EACHINCLUDING A SET OF HOLES IN STANDARDIZED LOCATIONS AND SAID MOUNTINGSTATIONS BEING ARRANGED IN A PATTERN SUCH THAT WHEN ALIGNED WITH THEHOLES OF A DRIVE PACKAGE FLANGE MOUNTED THEREON THE RECIPROCABLE RODSUPPORT WILL BE IN PRECISE ALIGNMENT WITH RESPECT TO A FUEL ELEMENTLOCATION ON SAID GRID; AND (C) RELEASABLE SECURING MEANS PASSED THROUGHTHE ALIGNED HOLES FOR SECURING MOTOR DRIVE PACKAGE MOUNTING MEANS INSELECTED MOUNTING STATIONS OF SAID PLATE.